Various attempts have been made to produce heat-insulated castings by enveloping ceramic parts in aluminum or cast iron castings. Since the modulus of elasticity of ceramics is considerably higher than that of metal and the coefficient of expansion of ceramics is lower than that of metal, compressive stress is developed in the ceramics and tensile stress in the metal at the boundary between the ceramic part and the metal as the molten metal solidifies and shrinks during cooling. Consequently, either the ceramic part is crushed or the metal yields or breaks down under tension. Usually the ceramic part, being weaker than the metal, is crushed. To avoid such crushing, the surface of the ceramic part which comes into contact with the metal is provided with a layer of a porous substance, which is compressed when the ceramic is enveloped in the casting, thereby preventing the ceramic part from being crushed. This method has, however, drawbacks in that when the layer of porous substance is too thick, the casting-enveloped ceramic part has a poor self-supporting strength and fails when subjected to slight vibration; and when the layer of porous substance is too thin, the ceramic part is crushed. In this method, depending on the properties of the layer of porous substance or the size and shape of the ceramic part, the appropriate thickness of the layer of porous substance is supposed to be 0.1-0.3 mm for, say, a cylindrical ceramic part about 50 mm in diameter. It is, however, extremely difficult to provide a layer of a porous substance on the ceramic part, with the thickness of the layer controlled to within such a narrow range.
Another method of producing heat-insulated castings is available, according to which a heat-insulating material is adhesively secured around a pipe of heat-resistant metal and the pipe is enveloped in a casting. In this case, however, the heat insulation obtained is not so good, because of substantial heat conduction from the pipe. Moreover, a heat-resistant metal of poor formability increases the cost of production, for it is exceedingly difficult to produce a pipe of intricate configuration therefrom.